Many suffering the side effects of chemotherapy have reported substantial relief (especially from nausea) after smoking or consuming cannabis. What is actually happening in these cases, and is this a helpful use of cannabis?

This is a helpful use of cannabis. It will vary in the degree of benefit from person to person but it is quite a real pharmacological effect and potential benefit. The mechanistic basis is that the nerve pathways governing nausea are also potentially blocked by the cannabinoids in the same way that nerve impulses associated with pain can be dampened by the cannabinoids, whether the natural endocannabinoids generated within the brain or exogenously administered cannabis used specifically for this purpose. The specifics of the nerve responses are what differentiate the relative efficacy and this is why the benefit for nausea is greater than that for pain, but there is some effect on both.